Travelling wave tube helix to coaxial line transition means



Jan. 19, 1960 J. B. KENNEDY TRAVELLING WAVE TUBE HELIX TO COAXIAL LINE TRANSITION NEANS v Filed June 3, 1958 INVENTOR JOSEPH B ENNEDY BY ATTORNEY United States Patent TRAVELLING WAVE TUBE HELIX T 0 COAXIAL LiNE TRANSETION MEANS Joseph B. Kennedy, Hicksvilie, N.Y., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Application June 3, 1958, Serial No. 739,530

5 Claims. (Cl. 3153.6)

The present invention relates to travelling wave tubes. It is particularly concerned with improved broadband impedance transition means for coupling the helixoi a travelling wave tube to input and output transmission line devices.

There are many kinds of transition sections employed in travelling wave tubes for attaining an impedance match between a helix and a coaxial line. One type section known in the art is constituted by a section of strip-above-ground plane transmission line. It includes a strip conductor wound in the form of a spiral having the same diameter as the helix, the width of the strip conductor being tapered. The narrow end of the strip conductor is connected to the helix, the wide end being connected to an inner conductor of a coaxial line to be matched to the helix. A ground plane conductor, which may be a horn, surrounds the spiral strip conductor. Both ends of the helix are respectively coupled to input and output coaxial lines through sections of strip-aboveground plane transmission lines as aforedescribed, for matching the impedance of the helix to the impedances of the coaxial lines.

The spiral strip conductors and helix are supported by a group of dielectric rods extending longitudinally of the travelling wave tube. The rods are contiguous with outer surfaces of the spiral and helix conductors, opposite ends of the rods being supported by the ground plane conductors at opposite ends of the tube.

A travelling wave tube utilizing transition sections between the helix and the input and output coaxial transmission lines as aforedescribed may not be as compact nor as rugged as is desired for some applications. Furthermore, the presence of the dielectric support rods between the spiral conductor and the ground plane conductor of each strip-above-ground plane transmission line constitute electrical discontinuities, which are generally desired to be avoided.

Therefore, it is an object of the present invention to provide a travelling wave tube having improved transition means for matching a helix to external transmission line devices.

It is a further object to provide a travelling wave tube.

having a helix coupled to an external transmission line by means including a spiral strip conductor ruggedly supported within a tubular ground plane conductor so that no dielectric discontinuities exist between the strip and ground plane conductors.

It is a further object to provide a travelling wave tube having one or more impedance transition sections which are readily constructed and supported on coaxial alignment with the helix and electron gun, and which occupy minimum radial and axial spaces in the tube.

The foregoing and other objects and advantages of this invention are attained by providing a helix having a pair of strip-above-ground plane transition sections coupled to the opposite ends thereof. Each transition section is composed of a strip conductor formed into a spiral having one end connected to the helix and the other end connected to a conductor of the transmission line to be matched. The spiral has a larger diameter than that of the helix, the width of the conductor form' ing the spiral being tapered. Both the helix and the spiral are supported within a tubular ground plane con-' ductor by a plurality of longitudinally extending dielectric rods disposed about the helix and within the spiral. Each spiral strip conductor and the tubular con ductor form a section of tapered transmission line for efiecting a smooth transition of microwave energy between the helix and coaxial line.

Referring now to the drawings,

Fig. 1 is a sectional view of a travelling wave tube including the novel impedance transition sections in accordance with the present invention;

Fig. 2 is a sectional view of the travelling wave tube taken along the lines 2-2 in Fig. 1; and

Fig. 3 is a sectional view of the travelling Wave tube taken along the lines 33 in Fig. 1.

Referring now to Fig. l, the numeral 11 designates an electron gun for producing and directing an electron beam along an axis of the travelling wave tube towards a collector electrode 12. The gun 11 consists of a cathode, not shown, a focussing electrode 13 and an anode 14. The electron gun is of a type for providing an electron beam which is initially convergent and electrostatically focussed to a minimum diameter at a predetermined location within an aperture 17 through the anode 14.

Although not specifically illustrated, it is understood that suitable permanent magnet or solenoid magnetic focussing means would be provided along the tube for ensuring that the beam has a constant diameter from the beam exit end of aperture 17 to the collector electrode 12. Both the anode 14 and the collector electrode 12 are formed from blocks of metal of highly magnetic material, and constitute pole pieces for the magnetic focussing system. The cathode and focussing electrode of the electron gun 11 are contained within a cavity in the anode block 14 of magnetic material so as to be shielded from magnetic lines of force. The principles upon which such a focussing is designed are known in the art, and need not be discussed. See US. Patent No. 2,707,758, issued May 3, 1955, to C. C. Wang, for example.

The cathode and focussing electrodes of the electron gun structure are coaxially supported in electrically insulated relationship from the anode by a dielectric disc member 21 afiixed to the end of a tubular vacuum envelope 22. The envelope 22 is vacuum sealed to a suitable recess provided in the block of metal forming the anode 14.

The collector electrode 12 is recessed at 25 for receiving the electrons of the beam after passage through the tube. The block of metal forming the electrode 12 is disposed within and vacuum sealed to the inner wall of one end of tubular conductor 26. The other end of tubular conductor 26 is supported by the anode block 14, and is brazed to a tubular member 23 extending from the anode in coaxial relationship therewith. Member 23 is vacuum sealed to an annular aperture provided in a face 27 of block 14.

A helix 29 for slow wave propagation of microwave energy is disposed within the tubular conductor 26 in coaxial relationship therewith. The helix 29 is sup ported by a plurality of dielectric rods 30, 31 and 32 of ceramic material, for example, extending longitudinally of the tube. The rods Bil-32 are contiguous with the outer surfaces of helix 29, and are angularly displaced about the axis of the tube by with respect 2,922,068 7 g e p 3 to each other. Suitable apertures are provided in the opposing faces 27 and 34 of the anode and collector electrodes, respectively, for receiving and supporting the rods 3ii3 2.

An input coaxial line having an outer tubular conductor 35 and an inner conductor 56 is provided at the anode end of the tube for supplying microwave energy to the helix. The inner conductor 36 extends into a" radial aperture 37 of cylindrical cross section provided in the anode block 14. The longitudinal axis of aperture37 is at right angles with the axis of the tube. The wall of the aperture 37 forms a continuation of the outer conductor of the input coaxial line. 1

A further aperture 38 of cylindrical cross section is provided in the anode block '14, the longitudinal axis of aperture 38 being parallel with the axis of the tube. The aperture 38 communicates with the aperture 37 as illustrated to form a further outer conductor portion of the input coaxial line.

A further inner conductor portion is formed by a rod 39 coaxially disposed within aperture 38. One end of the rod 39 is connected to the inner conductor 36 at right angles therewith. The characteristic impedance of the coaxial line formed by conductors 38 and 39 is the same as the characteristic impedance of the coaxial line formed by conductors 35-37. A portion of the rod 39 extends a short distance intofthe tubular conductor 26 for purposes which will become more clear further below.

A spiral conductor 42 having one end connected to the conductor 39 and another'end connected to the end of the helix 29 is located within the tubular conductor 26 to form part of an input impedance transition section between the coaxial line 3839 and the helix 29. The spiral conductor 42 is disposed about the three dielectric rods 3t}-32, and is supported thereby in coaxial relationship with the helix 29. It is preferable that the spiral conductor 42 be aflixed to the rods 3032 such as by brazing or glazing to form a rugged construction. The helix 29, spiral conductor 42 and rods 30-32 may be made to form a unitary structure for ready alignment of the helix and spiral conductor with the electron gun.

The spiral conductor 42 is formed by a thin strip of metal whose width is tapered from one end of the conductor to the other. This taper is illustrated as being linear, although it should be understood that a taper other than linear might also be used. The narrow end of the strip has a width dimension which approximates the diameter of the helix wire. An end portion 43 of the helix wire is brought out between the rods 3032 for connection to the narrow end of the strip conductor 42.

The spacing between the outer end of conductor portion 43 and the inner wall of conductor 26 is substantially the same as the spacing between spiral conductor 42 and conductor 26. The portion 43 is curved so that its spacing from the inner wall of conductor 26 increases gradually from its point of connection to the spiral conductor 42 to the region whereat the inner end of conductor portion 43 has the same spacing from conductor, 26 as the turns ofhelix 29. This is illustrated more clearly in Fig. 3.

The wide end of the spiral strip conductor 42 is connected to the rod 39. It is preferable that this connection be made as close to the face 27 of the anode 14 as mechanically possible.

The spacing between the turns of the spiral conductor 42 gradually decreases in the direction of the helix 29 for improving the transition of energy from the input coaxial line to the helix. The spacing between the first few turns of the helix 29 gradually decreases from a value which is approximately the same as'that between the immediately preceding turns of spiral 42, to a smaller value corresponding to the fixed spacing between the majority of the turns of the helix intermediate the end turns thereof. This is done for ensuring a better match over a wide frequency band.

The output coaxial line for the tube is formed by an outer conductor 45 joined to an end of the collector block 12 opposite an aperture 47 therethrough. The aperture 47 is of cylindrical cross section, and extends along the axis of the tube. This aperture constitutes a continuation of the outer conductor 45 of the output coaxial line. An inner conductor 46 of the output coaxial line is coaxially positioned within aperture 47, and extends into the tubular conductor 26 for a short distance.

A further spiral strip conductor 48 having one end connected to the inner conductor 46 of the output transmission line and another end connected to a conductor portion 49 forming an extension from the helix 29 is provided for forming an output impedance transition section similar to the input impedance transition section. The output impedance transition section is designed in the same manner as the input impedance transition section for matching the output coaxial line 46-47 to the helix 29.

The principles upon which the aforedescribed transition sections are designed will now be explained in greater detail. Since the input and output transition sections are substantially the same, only the input transition section will be referred to.

The helix 29 has an impedance relative to the conductor 26 which varies with frequency. Generally, this impedance is much higher than that of the input and output coaxial lines over the operating frequency range of the tube. In one tube which has been constructed for operation as a travelling wave amplifier over a frequency range of 2-4 kilorriegacycles, the helix has an impedance of the order of 200 ohms at an intermediate frequency within this range. Each of the input and output coaxial lines has an impedance of 50 ohms, which is frequency independent.

The various dimensions'of the helix 29 are idealized, in accordance with principles known in the art, for optimum travelling wave tube operation over the operating frequency band for the tube. The diameter of each of the dielectric rods 3fi32 is chosen to be approximately the same as the diameter of the helix 29. A one-to-one ratio between the diameter of the helix and the diameter of the dielectric supporting rod is known to be optimum. This will result in a minimum of dispersion of the helix impedance with frequency without unduly aifecting the helix energy due to the presence of the dielectric discontinuities. The inner diameter of tubular conductor 26 should be at least nearly twice the outer diameter of helix 29 for minimizing the effect of conductor 26 on energy carried by the helix in the helix mode.

The pitch of the end turns of the helix 29 is tapered to transform the impedance of helix 29 relative to conductor 26, which is frequency dependent, to approximately that of a wire-above-ground plane transmission line, which is frequency independent. The optimum pitch tap'er is best determined empirically. The wire-above-ground plane transmission line formed by conductor 26 and the end turns of helix 29, at the region where the inner end of conductor portion 43 is spaced from the inner wall of conductor 26 by h has an impedance of Z This is indicated in Fig. 3.

The end portion 43 of helix 29 is brought out between the dielectric rods 30 and 32 to a point spaced by a distance h from the conductor 26, as is also indicated in Fig. 3. The spacing between the turns of spiral 42 and the conductor 26 is also equal to k The conductor portion 43 and conductor 26 form a tapered section of wire-above-ground plane transmission line for gradually transforming the impedance Z to a lower impedance Z at the outer end of conductor portion 43, the impedance Z being considerably higher than that of the input coaxial line. The impedance aeaaoes where e is the dielectric constant of the medium between the conductors 43 and 29, h is the spacing between the outer end of conductor portion 43 nearest the conductor 26, and d is the diameter of conductor portion 43. Curving the conductor portion 43 so that its spacing from conductor 26 changes in a gradual manner from h to I2 enhances the impedance transition from Z to Z The impedance Z is a function of the spacing h between the spiral conductor 42 and the inner wall of the tubular conductor 26. Generally, if h is relatively large, the magnitude of the reflections from the junction between the spiral 42 and helix 29 will be minimized. Since the diameter of the rods 30-32 is substantially fixed by considerations already discussed, the diameter of the inner wall of tubular conductor 26 is the only variable for regulating h As has been mentioned before, the inner diameter of conductor 26 should be at least approximately twice the outer diameter of helix 29. Enlarging this diameter for increasing h to minimize reflections as aforedescribed, increases the coupling between the turns of spiral 42, unless the spacing between the turns of spiral 42 is also increased. Increasing the spacing between these turns adds to the axial length of the transition section. Generally, this length should be as small as possible. Therefore, the actual inner diameter of conductor 26 is reached by compromise, and is best determined empirically.

In one construction of a travelling wave tube amplifier for operation over a frequency range of 2-4 kilomegacycles, the diameter of each of rods 30-32 is slightly smaller than the outer diameter of helix 29. The inner diameter of tubular conductor 26 is slightly smaller than twice the outer diameter of helix 29.

The spiral strip conductor 42 together with the inner wall of tubular conductor 26 provides a tapered section of strip-above-ground plane transmission line. This transmission line section should have a sufiicient number of turns so that the efiective length of the taper, i.e., the length of the transmission line about the turns of conductor 42 from its point of connection to the inner conductor 39 to its point of connection to the conductor portion 43, is at least one-half wavelength at the lowest operating frequency for the travelling wave tube. The tapering of the pitch between the turns of spiral conductor 43 is best determined empirically.

The width of the conductor 42 at its narrow end is chosen so that the impedance of the strip-above-ground plane transmission line thereat is equal to Z The width of the narrow end of conductor 42 for providing an impedance equal to Z for the spacing 12 between conductors 26 and 42, can be determined from the design curves entitled Characteristic Impedance of Open Strip Line, contained in an article entitled Rigorous Determination of the Parameters of Microstrip Transmission Lines, by K. Black and T. Higgins, pages 93-113 of the March 1955 I.R.E. Transactions on Microwave Theory and Techniques.

The width of the strip conductor 42 at its widest end is similarly determined from the aforementioned curves for providing a strip-above-ground plane transmission line having an impedance which is equal to the character. istic impedance of the input coaxial line.

In operation of the aforedescribed tube, microwave energy over the desired operating frequency range is supplied to the input coaxial line 35, 36. This energy is efliciently transferred to the helix 29 by the input transition section with a minimum of reflections over the desired frequency range. After the energy has travelled to the output end of the helix 29, it is transferred to the output coaxial line 46, 47 by the output transition section. In one travelling wave amplifier tube which has been constructed for operation over a frequency range of 2-4 kilomegacycles, a standing wave ratio of two or less has been found to exist in the tube over this frequency range.

6 If the tube is used as an amplifier, the velocity of electron beam is adjusted, as is conventional in the art, to

, be approximately the same as the axial component of velocity of the travelling wave energy along the helix 26 from the end portion 43 to the end portion 49. If this is the case, interaction between the helix energy and the beam will occur to thereby amplify the helix energy in accordance with principles well known in the art. Although one type of travelling wave amplifier has been described, it should be apparent that an impedance matching transition section as described herein could be utilized equally well in other types of travelling wave tubes without departing from the scope of the invention.

It can be seen from the foregoing description that impedance transitions as aforedescribed constitute an improvement over prior art transition sections. No matching horns or superfluous structures are required as the inside surface of the tubular conductor 26, which constitutes the vacuum shell for the tube, serves as a ground plane conductor for the transition sections.

The helix, dielectric support rods 3032 and the spirals 42 and 48 are readily formed as a rigid subassembly for easy alignment with the electron gun, and support by the anode block of the gun. Since the inner diameter of each spiral transition section corresponds to that of a circle, circumscribing the-three dielectric rods 30-32, this diameter being slightly less than twice as large as the diameter of helix 29, the strip line occupies a much smaller axial space than if it had the same diameter as that of helix 29. Also, since each spiral strip line is supported on its inside by the dielectric rods 3032, no dielectric discontinuity is present between the spiral and ground plane conductors of the transition sections.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A travelling wave tube, comprising a tubular conductor that forms part of the vacuum envelope for said tube, a helix within said tubular conductor, means at one end of said conductor for producing and directing an electron beam through said helix, means at the other end of said conductor for receiving the electrons of said beam after passage through said helix, a plurality of dielectric rods disposed about said helix for support thereof within said tubular conductor, input and output coaxial transmission lines at opposite ends of said tubular conductor for connection to said helix, a first spiral conductor of larger diameter than said helix, said spiral conductor being connected to one end of said helix and an end of the inner conductor of the input coaxial line, and a second spiral conductor of larger diameter than said helix, said second spiral conductor being connected to the other end of said helix and an end of the inner conductor of said output coaxial line, said spiral conductors being disposed about and supported by said dielectric rods in coaxial relationship with said helix, each spiral conductor being composed of a strip conductor of tapering width that together with said tubular conductor provides a strip-above-ground plane transmission line for matching the impedance of said helix to the impedance of a respective one of said coaxial lines.

2. In a travelling wave tube the combination of a tubular conductor, a strip-like conductor in the form of a spiral that is coaxially disposed within said tubular conductor for providing a strip-above-ground plane transmission line, a travelling wave helix that is coaxially disposed relative to said spiral and connected thereto, the outer diameter of said helix being smaller than the inner diameter of said spiral, dielectric means extending along the outside of said helix and along the inside of said spiral for maintaining said spiral and said helix in coaxial relationship with each other, andmeansfor supporting lar conductor for maintaining said spiral"and'said helix in coaxial relationship with said tubular conductor.

3. The combination of claim 2 wherein said tubular conductor is part of a vacuum envelope for the travelling Wave tube.

4. The combination of claim 1 that further includes a block of metal for supporting the end of said tubular conductor nearest the beam producing means, an aperture extending from one to the other faces of said block for passage of the beam through the block, a further aperture that forms an L-shaped passage through the block from the perimeter of said block to its other face, means for supporting said input coaxial line upon said block at the opening in the perimeter of said block at one end of said further aperture, and conductive means supported Within said :further aperture for connecting the inner conductor of said input coaxial line to an end of said first spiral conductor, part of said conducting means extending into said tubular conductor in adjacent relationship with the inner surface of said tubular conductor and in substantially parallel relationship with the axis of the tube, said part of said conducting means being spaced from said tubular conductor by substantially the same amount as said first spiral conductor andbeing connected directly to an end turn of said first conductor.

the same amount as said second spiral conductor and 2,922,oes

5. The combination of claim 4 that further includes another block of metal for supporting the other end of said tubular conductor nearest the beam receiving means,

an aperture that extends through the other block, means for supporting said output coaxialline upon said other block at the opening at one end of'the aperture therethrough, and conductive means supported within said further aperture for connecting the inner conductor of said output coaxial line to an end of said second spiral conductor, part of the last-named conductive means extending into said tubular conductor in adjacent relationship with the inner surface of said tubular conductor and in substantially parallel relationship with the axis of the tube, said part of said last-named conductive means being spaced from said tubular conductor by substantially being connected directly to an end turn of said second conductor.

References Cited in the file of this patent UNITED STATES PATENTS 2,637,775 Lund May 5, 1953 2,727,179 Lalley et al. Dec. 13, 1955 2,788,465 Bryant et al. Apr. 9, 1957 2,794,936 Huber June 4, 1957 2,822,492 Burke Feb. 4, 1958 

